INTRODUCTION

The glenohumeral joint, due to its high mobility, is particularly susceptible to dislocations. Anterior shoulder dislocations are relatively common, but it is rare for them to be irreducible, especially when caused by the entrapment of a rotator cuff muscle (RCM). We present a case where glenohumeral reduction failed due to the interposition of the subscapularis muscle (SSM), which was also accompanied by the avulsion of its osseous insertion from the lesser tuberosity of the humerus (LTH). This case highlights the often “misleadingly reassuring” appearance of postreduction radiograph, which can overlook the entrapment of one or more anatomical elements, either bony and/or soft tissue, thereby delaying surgical intervention and potentially leading to dramatic functional consequences in typically young patients.

CASE REPORT

A 35-year-old right-handed man who worked as a manual laborer presented to the emergency department after falling approximately 2 m and landing on his left shoulder. The clinical examination revealed a noticeable deformity and swelling, shortening of the left arm, and a subacromial void. There was a clear anterior glenohumeral dislocation with the left upper limb in an abduction-external rotation position. Both humeral and radial pulses were present and symmetrical. No neurological deficits were detected in the territories of the ulnar, median, and radial nerves, or in the axillary nerve territory (sensation in the shoulder stump was preserved). The rest of the musculoskeletal system examination was unremarkable. The initial imaging assessment consisted of a single anteroposterior radiograph taken with the patient in a supine position. It showed a glenohumeral dislocation with two bony fragments, initially thought to be from a comminuted fracture-avulsion of the greater tuberosity of the humerus (GTH) (Fig. 1A).

The emergency team performed a reduction under sedation, and follow-up radiographs, also in a supine position, revealed a partial reduction of the dislocation. The anteroposterior view showed significant overlap at the joint space, and the lateral view displayed subluxation of the humeral head with the two bony fragments positioned posteriorly (Fig. 1A, B), indicating clear joint incongruity. A computed tomography (CT) scan confirmed that the two posterior fragments were, for the lateral-most fragment, a comminuted fracture-avulsion of GTH, and for the more medial fragment, a fracture-avulsion of the LTH. The soft tissue window sequences revealed that the latter remained tethered to the tendon of the SSM, which was outstretched and incarcerated between the glenoid cavity and the humeral head, preventing satisfactory reduction. The comminuted fragment detached from the GTH retained its attachments with the tendons of the SSM and infraspinatus muscle. A Hill-Sachs impaction was also observed (Fig. 2).

The patient was admitted to the operating room and placed in the beach chair position under general anesthesia. A deltopectoral approach was used, navigating between the deltoid and pectoralis major muscles while preserving the cephalic vein. After incising the clavipectoral fascia, joint exploration facilitated by the already induced capsular opening from the dislocation allowed identification of the anteriorly subluxated humeral head and the fracture zones voided by detached fragments from both humeral tuberosities (Fig. 3A). The SSM, visibly entrapped between the glenoid cavity and the humeral head, was released along with the fragment of the LTH that remained attached to it (Fig. 3B), enabling immediate and easy glenohumeral reduction. The fragment of the GTH was also identified; it was comminuted but remained attached to the SSM tendon and part of the infraspinatus tendon. A tenotomy of the long head of the biceps tendon (LHBT), which was torn, was performed, and tenodesis of the tendon was performed to the soft tissues around its groove at the high humeral insertion of the pectoralis major. After reducing the lesser tuberosity fragment, it was fixed with two 4.5-mm cannulated screws (Figs. 3C, 4). Due to the comminution of the greater tuberosity fragment, fixation was achieved using a series of three longitudinal transosseous sutures with nonabsorbable 2-mm diameter thread.

Stability tests were satisfactory. Postsurgery, the patient wore a sling immobilizing the elbow to the body for 3 weeks, allowing regular passive mobilization of the elbow and wrist, followed by functional rehabilitation. At the 12-month follow-up, the patient achieved a Constant-Murley score of 85 out of 100, with a limitation in internal rotation at L3 but no signs of instability or new dislocation episodes. He continues to work as a manual laborer.

Ethics statement

This case report was approved by the Scientific Committee and the Medical Council of Centre Hospitalier de Gonesse (No. CHG/23-012). Written informed consent for publication of the research details and clinical images was obtained from the patient. The study was conducted in accordance with the principles of the 1964 Declaration of Helsinki and its later amendments.

DISCUSSION

The failure to reduce an anterior shoulder dislocation can be attributed to various mechanical blocks caused by the interposition of bony elements and/or soft tissues. The literature on this subject is disparate and primarily consists of case reports, making it challenging to group patients into homogeneous cohorts and draw practical conclusions based on correlations among the dislocating mechanism, imaging findings, and surgical observations. Most often, irreducibility results from entrapment of the LHBT alone or in association with a fracture of the GTH [1,2]. The interposition of one or more RCM or their tendons [3], particularly the subscapularis muscle [3–8], is the second most common cause. Other factors that can block reduction include interposition of the labrum [9], a displaced fractured fragment from GTH or glenoid, a Hill-Sachs bone impaction lesion fixed on the glenoid rim [10], a bony Bankart lesion [11], and possibly the interposition of the musculocutaneous nerve [12]. The SSM is thus the RCM most frequently involved in blocking glenohumeral reduction, even though the injury mechanism of anterior dislocation often results in associated damage to other cuff muscles, particularly in individuals over 40 years old [13].

Repairing these muscles should be considered as part of the operative strategy. This may involve avulsion of the subscapularis tendon insertion at the tuberosity, where the tendon and/or muscle part can project into the glenohumeral space (GHS), or an avulsion-fracture that additionally involves a more or less significant portion of the LTH, with the bony fragment becoming incarcerated in the GHS or completely posterior to the humeral head. In most cases, a Hill-Sachs impaction is found. Bruce-Brand et al. [4] described a case of subscapularis incarceration that went unnoticed for 3 weeks following an anterior shoulder dislocation that was initially considered reduced, but subsequently manifested with significant and unusual pain, muscle weakness, and mobility limitation. Miles et al. [5] also presented a case of an incarcerated subscapularis with the LHBT in the GHS, unnoticed during the initial reduction of an anterior dislocation, after which the shoulder subsequently “did not improve.” Diagnosed at 3 weeks postinjury with a CT scan and magnetic resonance imaging (MRI) revealing a massive rotator cuff tear, the patient underwent surgery in the sixth week. Bridle and Ferris [6] described the intraoperative diagnosis of subscapularis entrapment, where clinical suspicion of soft tissue interposition was quickly established after reduction maneuvers by obtaining a simple lateral radiograph confirming persistent irreducible anterior subluxation, prompting surgical intervention. Connolly et al. [7] also presented a case of delayed diagnosis in the second week, in a patient who initially had an anterior shoulder dislocation that was considered reduced based on radiological findings, but whose immediate clinical course was unfavorable, prompting reassessment and additional CT/MRI scan revealing entrapment of the subscapularis and LHBT associated with a massive RCM tear. Even closer to the lesion description in our case, Walch et al. [3] reported the case of a patient whose anterior dislocation was considered reduced and immobilized for 3 weeks, followed by functional rehabilitation sessions without improvement for 7 months before the diagnosis was finally made. It involved incarceration of the subscapularis and its bony attachment, which was identified as a periosteal avulsion of the LTH. The biceps tendon was also dislocated within the joint, and the remainder of the rotator cuff was ruptured. More recently, another case of subscapularis interposition and its bony attachment involving the LTH was described by Ayoubi et al. [8]; in that case, these elements formed a significant barrier of interposition between the humeral head and the fractured fragment of the GTH, preventing reduction of the dislocation.

Although it is challenging to establish a single pathomechanical model for the sequence of forces leading to subscapularis interposition and the occurrence of all associated injuries, one explanation proposed to account for this irreducibility—likely representing a certain percentage of cases of subscapularis avulsion-fracture entrapment associated with supraspinatus avulsion-fracture—has been proposed by Wolf et al. [14]. They argue that anterior dislocation, in some cases, may actually result from an initial inferior dislocation causing significant tension on the tuberosity insertions of the supraspinatus and subscapularis, with or without fracture, driving the humeral head anteriorly to the latter, thereby inducing reduction blockage. Furthermore, subscapularis incarceration—with or without fracture of the LTH—is not limited to anterior shoulder dislocation but can also occur with posterior dislocation [15].

While numerous studies have already drawn the medical community’s attention toward the need to recognize specific radiological signs of posterior dislocation to avoid oversight, it is equally important not to overlook subtle radiological signs which, when combined with clinical findings, should raise suspicion of glenohumeral interposition on a “misleadingly reassuring” postreduction radiograph following anterior shoulder dislocation. Indeed, in many cases, when a postreduction anteroposterior radiograph is taken, there is a misleadingly reassuring “pseudo-reduced” appearance where the dislocation appears improved, but with subtle signs of subluxation, including upward and lateral displacement of the humeral head, resulting in reduced subacromial space and widened joint space. However, if the radiograph is not taken in strict alignment, the widened joint space may be replaced by an incongruity with overlap at the joint line, making the diagnosis of interposition even more challenging. Clinically, however, the findings of persistent sharp pain alongside significant limitation in range of motion should always draw attention. The patient, who must always be listened to, is often the best aid to diagnosis, as, in addition to reporting pain, he or she may describe a “sense” of discomfort, a “feeling” of the shoulder not being entirely in place, and a “sensation” of joint discomfort despite reduction maneuvers and a seemingly normal radiograph. Therefore, it is crucial not to rely solely on the anteroposterior view and to confirm the diagnosis by systematically performing at least a lateral view, which can help identify persistent anterior subluxation.

The clinician should not hesitate to use CT or MRI scans at the slightest doubt, especially as these increasingly accessible exams enable a precise diagnosis of the interposed elements and assessment of associated injuries including bone (tuberosities fractures, Hill-Sachs lesion, and bony Bankart lesion) and soft tissues (rotator cuff muscles, long head of the biceps, and labrum). This approach allows prompt refinement and optimization of the therapeutic strategy. Any diagnostic delay compromises the chances of effective surgical intervention and leads to dramatic functional consequences, particularly in often young patients. The important message we would like to highlight through this case report is the importance of confirming dislocation reduction on at least two orthogonal views, being attentive to the patient's impressions regarding the feeling of shoulder reduction, and considering a CT or MRI scan if there is any doubt. The deltopectoral surgical approach, de-entrapment, and repair of the rotator cuff muscles remain standard practice.

ARTICLE INFORMATION

Conflicts of interest

The authors have no conflicts of interest to declare.

Funding

The authors received no financial support for this study.

Author contributions

Conceptualization: NS; Methodology: all authors; Investigation: all authors; Writing–original draft: NS; Writing–review & editing: all authors. All authors read and approved the final manuscript.

Data availability

Data sharing is not applicable as no new data were created or an¬alyzed in this study.

Fig. 1.

Initial radiograph images of the patient. (A) Anteroposterior radiograph demonstrating a glenohumeral dislocation with two bony fragments. (B) Postreduction anteroposterior view demonstrating a significant overlap at the joint space. (C) Postreduction lateral view demonstrating subluxation of the humeral head with the two bony fragments positioned posteriorly. These are poor radiographic views obtained with the patient in a supine position.

Fig. 2.

Computed tomography (soft tissue window sequences) demonstrating anterior subluxation of the humeral head and the entrapment of the lesser tuberosity and subscapularis muscle, which was outstretched and incarcerated between the glenoid cavity and the humeral head, preventing satisfactory reduction. The comminuted fragment of the greater tuberosity maintains its attachments with the tendon of the infraspinatus muscle. A Hill-Sachs impaction can also be observed.

Fig. 3.

Intraoperative photographs. (A) Insertion site of the lesser tuberosity. (B) Release of the subscapularis muscle and the lesser tuberosity remaining attached to it. (C) Reduction and fixation of the lesser tuberosity.

Fig. 4.

Postoperative radiographic and computed tomography imaging. (A, B) The reduction of the two tuberosities and the placement of two screws for the fixation of the lesser tuberosity. (C) Three-dimensional reconstruction demonstrating the repositioning of the greater tuberosity fragment and the fixation of the lesser tuberosity with two screws.

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